1. Field of the Invention
The present invention relates to a method and an apparatus for treating gas such as flue gas by irradiation of electron beam, and more particularly to a method and an apparatus for treating gas containing sulfur oxides and/or nitrogen oxides by injection of ammonia and irradiation of electron beam to thereby remove sulfur oxides and/or nitrogen oxides therefrom.
2. Discussion of the Background
As economy develops, more and more energy is demanded. Amidst the continuous growth of energy demand, energy source is still dependent on fossil fuels such as coal and petroleum. However, the harmful products or pollutants generated by burning of fossil fuels are responsible for global pollution. To prevent the release of pollutants into the atmosphere and to stop the pollution of global environment, development work is being carried out at an accelerated pace to create a flue gas treatment system for installation in fuel combustion plant such as thermal power plants. There are still many areas of improvement to meet problems such as the complicated configuration of the equipment requiring a large number of control variables and the need for large-scale waste water treatment systems requiring sophisticated treatment technology.
In an effort to solve these problems, a flue gas treatment system in which flue gas discharged from the fuel combustion facility such as a boiler is treated by irradiation of electron beam has been developed.
In this system, ammonia is injected into gas containing sulfur oxides and/or nitrogen oxides, and the mixed gas is irradiated with electron beam to remove the sulfur oxides and/or the nitrogen oxides therefrom in the form of reaction by-product (hereinafter referred to as by-product). However, this by-product mainly composed of ammonium sulfate and/or ammonium nitrate tends to be deposited on a passage for allowing the treated gas to pass therethrough. There have been made various attempts to prevent deposition of by-product mainly composed of ammonium sulfate and/or ammonium nitrate in such an electron-beam process.
According to one of the attempts, the speed of a gas flow after irradiation of electron beam is set to 10 m/s or less, preferably 5 m/s or less, as disclosed in Japanese patent publication No. 3-65211. According to another attempt, the temperature of a gas-contacting portion from an electron-beam irradiation area to a by-product collecting area is kept in a range of 80 to 150.degree. C., preferably 100 to 150.degree. C., as disclosed in Japanese laid-open patent publication No. 7-31844.
The former attempt is disadvantageous in that an apparatus downstream of irradiation area of electron beam and/or a cross-sectional area of a duct need to be large, resulting in a large-sized facility. The latter attempt is problematic as it requires a heat source for raising the temperature of the gas-contacting portion. Further, both attempts fail to completely prevent the deposition of by-product, but allow by-product to be deposited at an accelerated rate once it starts being deposited.
It has been proposed in International application No. PCT/JP97/00772 to surround a passage having inlet and outlet openings with a shielding wall and place partition plates and/or inner partition walls in the shielded passage in order to shield X-rays that are generated by irradiation of electron beam. If the proposed structure is applied to a duct extending from a process vessel to a by-product collector in the flue gas treatment system, then a portion of the duct is constructed as the shielded passage and the partition plates and/or inner partition walls are provided in the shielded passage. Generally, it is known that the by-product tends to be deposited in locations where the gas flow is turbulent. Consequently, in the proposed structure, since the gas flow is caused to be turbulent by the partition plate and/or the inner partition wall in the shielded passage, the by-product will be deposited at a greater rate on the partition plate and/or the inner partition wall or thereabouts.